The Soil Hollow Cylinder Test was developed to allow for independent control of all principal stresses seen by soil in-situ, including the σ2 intermediate stress, which is generally ignored in standard soil triaxial tests and true triaxial tests. The hollow cylinder test is commonly used to determine the dynamic properties of soil, such as liquifaction potential, dynamic shear strength, and damping, and can also be used to perform standard static triaxial tests. The hollow cylinder test is performed by applying three different stresses to a hollow, cylindrical soil specimen. These three stresses can each be individually controlled, like in the true triaxial test. However, in the hollow cylinder test, the stresses are applied outside the specimen (the confining stress), inside the specimen, and a torsional stress at the top of the specimen. By controlling each of these three stresses (the principal stresses) during the test, the effects of different stress paths on the shear strength of the soil can be determined.
The torsional stress can also be applied as a dynamic stress instead of a static stress in order to determine the shear modulus of a soil. The torsional stress is usually applied at relatively low frequencies with relatively high magnitudes when compared to a resonant column test or the torsional shear test.
Hollow Cylinder Cell (Courtesy of GCTS)
A typical specimen is loaded onto a platen inside a triaxial cell and another platen is placed on top of the specimen. An axial load piston is brought in contact with the top platen and the triaxial cell is filled with a confining liquid. While the triaxial cell is being filled, the pressure inside the specimen should be controlled to ensure the specimen does not collapse. Once the triaxial cell is full, water is allowed to enter the specimen, causing it to become completely saturated. The axial load is then slowly and incrementally increased to allow the specimen to consolidate.
After consolidation, the confining, internal, and axial load are set for testing. The axial load is then rotated, causing a torsional shear stress at the top of the specimen while the bottom of the specimen remains stationary. If performing a triaxial test, the top of the specimen is rotated at a set rate until failure occurs. This test can be performed with different levels of axial, confining, and internal stresses, and all of these can be changed while testing occurs, which allows for an analysis of how different stress paths affect the shear strength of the soil.
If performing a test to determine the shear modulus of the soil, the torsional stress is applied and removed at a relatively low frequency (up to about 10 Hz), but the specimen is forced to rotate significantly with each application (up to approximately 25°). This allows for a determination of the shear modulus of the soil at the given strain value, which can be combined with the resonant column test and a torsional shear test to produce most of the shear modulus versus strain curve for a soil.
The specimen used for a hollow cylinder test is typically made in a two-ring mold. The outer ring controls the outer diameter of the specimen and the inner ring controls the inner diameter of the specimen. These specimens are typically around 100 mm (3.94 in.) in outer diameter, 60 mm (2.36 in.) in inner diameter, and 200 mm (7.87 in.) in height. The specimen is wrapped in membranes to prevent the confining liquid from entering the specimen during testing.
In order to determine the shear strength, angle of internal friction, and cohesion of the soil, a Mohr’s Circle Analysis must be performed, similar to the soil triaxial test. However, since there are three different stresses acting on the specimen, all three stresses must be plotted. A graph should be made with principal stresses on the x axis and shear stress on the y axis. As described in the soil triaxial test, each of the three principal stresses should be plotted with zero shear stress. Three circles should then be drawn, with each set of points acting as the endpoints of the circle. Put simply, the first principal stress and the second principal stress should be the endpoints of a circle, the second principal stress and the third principal stress should be the endpoints of a circle, and the first principal stress and the third principal stress should be the endpoints of a circle. The radius of the largest circle is equal to the shear strength of the soil in that stress state. The largest circle is also used to determine the angle of internal friction and the cohesion of the soil as described in the soil triaxial test. The two smaller circles serve to describe the stress state of the soil, as different stress states will yield different values for the shear strength, angle of internal friction, and cohesion of the soil.
The process for determining the shear modulus of the soil is very similar to the process used after the torsional shear test. By following this procedure, the shear modulus of the soil can be determined for the strain value used during testing.
The hollow cylinder test apparatus has a few major advantages over other related systems. It allows for true triaxial testing, which gives it an advantage over a typical triaxial system. There is only one loading piston on the hollow cylinder system, so it is less complex than a true triaxial system, which has either four or six loading pistons. Since the system can also perform shear modulus tests, it allows for a wider range of testing than just a triaxial system.
However, the hollow cylinder test system does come with a few drawbacks. It is a much more complicated system than a typical triaxial system and requires much more control to use. Therefore, it is used most often for research. In addition, specimen preparation requires much more time and care, as a hollow specimen is much more difficult to create than a solid specimen used in a triaxial test.
Keywords: Soil Hollow Cylinder — Hollow Cylinder Test — Soil Hollow Cylinder Testing System — Soil Hollow Cylinder Test Machine — Hollow Cylinder Testing Instrument — Soil Hollow Cylinder Tester — Angle of Internal Friction — Cohesion — Shear Strength — Consolidation
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